Complete "Drop python2 support" goal

We stopped testing python2, but there are more things to be cleanup.

- Remove python2 entries from setup.cfg
- Add releasenotes
- remove universal wheel since this is only python 3 now

Change-Id: Ie2bbb4d34b8411939ad5cfd750fc76c933779542

doc/requirements.txt

@@ -3,8 +3,7 @@
 # process, which may cause wedges in the gate later.
 openstackdocstheme>=1.23.2 # Apache-2.0
 reno>=2.5.0 # Apache-2.0
-sphinx!=1.6.6,!=1.6.7,>=1.6.5,<2.0.0;python_version=='2.7' # BSD
-sphinx!=1.6.6,!=1.6.7,>=1.6.5;python_version>='3.4' # BSD
+sphinx!=1.6.6,!=1.6.7,>=1.6.5 # BSD
 sphinxcontrib-apidoc>=0.2.0 # BSD
 
 # redirect tests in docs

releasenotes/notes/drop-py2-421c90fbdf18dbc2.yaml

@@ -0,0 +1,7 @@
+---
+upgrade:
+  - |
+    Python 2.7 support has been dropped. The last release of
+    python-openstackclient to support Python 2.7 is OpenStack Train. The
+    minimum version of Python now supported by python-openstackclient is
+    Python 3.6.

setup.cfg

@@ -13,8 +13,6 @@ classifier =
     License :: OSI Approved :: Apache Software License
     Operating System :: POSIX :: Linux
     Programming Language :: Python
-    Programming Language :: Python :: 2
-    Programming Language :: Python :: 2.7
     Programming Language :: Python :: 3
     Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.7
@@ -720,9 +718,6 @@ openstack.volume.v3 =
 [upload_sphinx]
 upload-dir = doc/build/html
 
-[wheel]
-universal = 1
-
 [extract_messages]
 keywords = _ gettext ngettext l_ lazy_gettext
 mapping_file = babel.cfg